GST Invoice Format: What Every Small Business Actually Needs

A GST invoice isn't just a bill with a tax percentage added — it's a specific document with fields the law requires, and getting them wrong can cause real problems at filing time. Here's what actually has to be on it.

The fields that must be there

• Invoice number and date — sequential, unique for the financial year.
• Supplier's name, address, and GSTIN.
• Customer's name and address, plus GSTIN if the customer is registered.
• HSN or SAC code for the goods or service — this determines the applicable tax rate and is often the field businesses skip or get wrong.
• Description, quantity, and unit of what's being supplied.
• Taxable value — the amount before tax.
• CGST, SGST, or IGST — rate and amount, shown separately.
• Place of supply, especially important for interstate transactions where IGST applies instead of CGST+SGST.
• Signature of the supplier or an authorised representative (a digital signature is acceptable for electronic invoices).

CGST + SGST, or IGST — how to tell which applies

This trips up a lot of small businesses. The rule is simple once you know it:

• Same state (supplier and place of supply in the same state) → CGST + SGST, split evenly. A 12% item charges 6% CGST + 6% SGST.
• Different states (interstate supply) → IGST at the full combined rate. A 12% item charges 12% IGST, not split.

Getting this wrong — charging CGST+SGST on an interstate sale, for instance — is one of the more common invoicing errors and can cause mismatches when returns are filed.

Mistakes worth double-checking

• Wrong or missing HSN/SAC code — this is the single most common error, since the rate depends on getting the right code.
• Rounding tax off inconsistently across CGST and SGST lines rather than the total.
• Missing the customer's GSTIN when they are registered — needed for them to claim input tax credit.
• Non-sequential invoice numbers, which can raise questions during an audit.

What about quotations, purchase orders, and receipts?

These don't need the same strict GST-invoice fields since they aren't tax documents, but keeping a consistent, professional format across quotations, invoices, and receipts makes a small business look established — and makes your own bookkeeping much easier to follow later.